Intracellular delivery of SIV expression vectors into the epidermis of rhesus macaques using the AccellTM gene-gun resulted in in the induction of significant IgG responses. These responses were dramatically elevated following a booster immunization with a recombinant vaccinia virus. Three monkeys primed with several consecutive doses of DNA encoding SIVmac239 gp120 and gp160 developed maximum IgG titers of approximately 1:20,000. Following a single booster immunization with a live recombinant vaccinia virus expressing SIVmac239 and gp160, these animals developed transient peak endpoint titers of >1:2x106. These responses were dramatically higher than those seen in animals primed and boosted with either DNA or vaccinia vectors alone. In addition, a group of animals that were first primed with a vaccinia virus expressing either SIVmac239 gp160, or SIV deltaB670 gp160 or a combination of both, also developed similar responses following a single booster immunization with DNA, demonstrating that the synergistic effect is not dependent on the order of vaccine administration. All vaccinates and 5 controls were challenged 19 weeks after the booster immunization by intravenous challenge with 10 minimum infectious doses of SIVdeltaB670. Vigorous antibody responses after the booster immunization were not persistent and had fallen to titers of approximately 1:3,000 to 1:35,000 at the time of challenge. All animals were infected as determined by PCR of PBMC DNA14 days post-challenge demonstrating the vaccines did not induce sterilizing immunity. Virus loads, neutralizing antibodies, PCR, plasma antigenemia, ELISAs, and clinical examinations were employed to monitor the level of protection post-challenge.